Which of the following anion has the lowest hydration enthalpy?

The correct option is D $I^{-}$
$\text{Hydration enthalpy}\propto \frac{\text{Charge on ion}}{\text{Size of ion}}$
Since the charge on the anions are the same hence smaller the ion, higher the hydration enthalpy.

As we move down the group, the size of halogens increases. Thus, $I^{-}$ will have the lowest hydration enthalpy.

Anion	Hydration Enthalpy (in kJ/mol)
$F^{-}$	515
$C{l}^{-}$	381
$B{r}^{-}$	347
$I^{-}$	305